A person in California has tested positive for a form of mpox causing a widespread epidemic in Africa, the state’s Department of Public Health reported on Saturday. It is the first known case in the United States.

The individual, who was not identified, had recently returned from East Africa. The patient was diagnosed in San Mateo County, just south of San Francisco, and was isolating at home.

Officials at the California Department of Public Health and at the Centers for Disease Control and Prevention are reaching out to potential contacts of the patient for further testing.

There is no evidence that this version of the mpox virus, called Clade Ib, is circulating in communities in the United States, C.D.C. officials said.

Infections in people returning from Africa, however, have been found in Germany, Sweden, Thailand and the United Kingdom, among other countries. A case in India was reported in a person returning from the United Arab Emirates.

In Germany, Sweden, Thailand and India, the virus was transmitted no farther. In the United Kingdom, the infected individual passed mpox to three household contacts.

“The anticipated overall risk of Clade I mpox to the general population in the United States from the outbreak in Central and Eastern Africa is low,” the C.D.C. said in a statement on Saturday.

There are two main types of mpox, formerly monkeypox: Clade I and Clade II. A subtype of Clade II caused a global epidemic in 2022, mostly among gay and bisexual men, affecting nearly 100,000 people in 116 countries. About 200 people died.

That virus continues to circulate in the United States, but now at lower levels.

More recently the Clade Ib subtype has caused enormous misery in the Democratic Republic of Congo, as well as in Burundi, Central African Republic, Republic of Congo, Rwanda and Uganda.

There have been more than 57,000 confirmed or suspected Clade I cases this year and nearly 1,200 deaths, many of them children. The World Health Organization declared the epidemic a global emergency in August.

Until recently, Clade Ib spread mainly through consumption of contaminated meat or close contact with infected animals and people. But last year the virus was discovered to be spreading sexually, often through heterosexual prostitution.

Mpox is transmitted through close personal contact. The incubation period is three to 17 days, and the illness usually manifests as a painful rash on the hands, feet, chest, face, anus or genitals.

The C.D.C. recommends that all people at risk, including sexually active gay and bisexual men, receive two doses of the Jynneos vaccine.

Apoorva Mandavilli contributed reporting.